Batteries as what you pay for, if you have a free source of batteries I would just use that as your battery type (ie while I was in the Australian army I could grab lithium AA and AAA batteries when ever I felt sneaky, I havnt seen anyone's helmet using the battery packs in RC cars, maybe that's because of weight, the next gen of RC helicopters batteries are getting lighter, and some are smaller, a RC store would have the Chargers, you would need to build a circuit no matter what kind of rechargeable batteries you used to deal with stopping the batteries going to a zero charge state (that kills your batterie )